1. Field of the Invention
The present invention relates to a gas sensor, and, more particularly, to a gas sensor which can be inhibited from decrease in output of the gas to be measured at the time of detection and besides can attain stable output of the gas to be measured even after use of long time.
2. Description of the Prior Art
From the viewpoints of environmental protection such as inhibition of warming of the earth or inhibition of atmospheric pollution, gas sensors are used for detecting and measuring the concentration of various gases contained in exhaust gases discharged from internal combustion engines such as engines of automobiles, combustion exhaust gases discharged from combustion apparatuses such as combustion furnaces and incineration furnaces, e.g., oxygen (O2), carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2), nitrogen oxides (NOx such as NO and NO2) sulfur dioxide (SO2), water (H2O), etc.
The gas sensors include, for example, oxygen sensors used for measuring oxygen concentration in exhaust gases of automobiles to detect the combustion state in the engines (JP-A-9-68515). Recently, for the improvement of performance of automobiles, it is attempted not only to enhance performance of engines, but also to add various additives such as phosphorus, zinc, magnesium and calcium to engine oil or gasoline. However, if these additives mingle into exhaust gases, they stick to the surface of oxygen sensor to clog the communicating pores of the diffusion resistant layer or stick to the measuring electrode to deteriorate the electrode, resulting in reduction of output of the sensor or reduction of responsiveness.
Furthermore, there are NOx gas sensors for measuring NOx gas concentration in the exhaust gas in which the measuring electrode is protected with an electrode protective layer (JP-A-2003-322636). Porous alumina sintered bodies are generally as the electrode protective layers. The electrode protective layer has a function of diffusion regulating means for the gas to be measured. Moreover, since the measuring electrode is composed of an electrode material containing, for example, platinum (Pt) and rhodium (Rh) as main component metals, the measuring electrode made using an electrode material containing platinum (Pt) and rhodium (Rh) as main component metals has the problem of being poisoned with a slight amount of Au volatilized and scattered at a sintering step to cause considerable reduction of NOx gas decomposition activity, and hence an electrode protective layer comprising a porous alumina sintered body is formed on the surface of the measuring electrode.
The inventors have found that the NOx sensors have such an additional problem that during the use of them, gold (Au) volatilizes and scatters from an electrode of pump cell due to high temperature and sticks to the measuring electrode (e.g., a Pt—Rh electrode comprising platinum (Pt) and rhodium (Rh)) and reacts (poisons the measuring electrode) to cause deterioration of NOx gas decomposition activity (decrease in NOx gas output).
The porous alumina sintered body used as an electrode protective layer has been produced, for example, by preparing a paste comprising a mixture of an alumina powder as an aggregate and an aluminum salt as a pore forming material at a specific ratio, forming the paste into a desired shape and then sintering the formed body. When the alumina sintered body is used as an electrode protective layer for a measuring electrode of NOx gas sensors for detecting NOx gas in an exhaust gas discharged from automobiles, this is made by forming the prepared paste into a layer or thin film by screen printing and then sintering, and there are problems that variations occur in pore diameter and porosity of the resulting alumina sintered body (electrode protective layer), and NOx gas sensor of high accuracy cannot stably be made. Furthermore, since the input NOx gas must permeate the electrode protective layer, reach the measuring electrode and react to stably output the decomposed N2 and O2−, the layer is required to have uniform porosity higher than a specific value, and, besides, in order to inhibit the deterioration of NOx gas decomposition activity (reduction in NOx gas output) caused by penetration of gold (Au) volatilizing and scattering from the pump electrode through the electrode protective layer and sticking of the gold to the measuring electrode to cause reaction (poisoning of the measuring electrode), the layer is required to have a uniform pore diameter smaller than a specific value. In other words, the electrode protective layer is required to have intermediate physical properties between those of porous body and dense body.
FIG. 8 is a graph which shows relation between pore diameter (Å) of the electrode protective layer and Nox gas output reduction rate (%) which is an indicator for high-temperature endurance when a NOx gas sensor provided with a conventional electrode protective layer was continuously operated at 950° C. for 100 hours. As can be seen from FIG. 8, when the pore diameter of the electrode protective layer exceeds 1100 Å, the NOx gas output reduction rate (%) sharply increases.